This invention relates in general to the field of graphic arts and more particularly to the registration of halftone color separation film. Still more particularly, the invention is directed to a method and apparatus for obtaining highly accurate registration of color separation films in order to permit printing of high quality color pictures.
In the printing of high quality color photographs such as those which appear in mass circulation magazines and other publications where quality is of overriding importance, the color original is broken down into separate photographic images that are processed separately. The printed picture is a halftone picture in which the appearance of continuous tones is realized by breaking the picture detail up into halftone dots of varying sizes. In the commonly used four color process, the halftone color separations are four in number. Each color separation carries black and white tone information that reflects the process color content of the original color picture. The final reproduction requires that the four halftone color separation films be precisely registered or aligned so that the printed picture faithfully reproduces the original.
At present, process color registration is for the most part performed manually by highly trained persons referred to in the printing industry as strippers. The manual registration process involves taping or stripping the separation films (negatives or positives) to carrier sheets which are usually clear polyester film. The first reference color film is stripped to a pre-punched carrier sheet which is often the size of 4, 8, 16 or 32 pages. The film with the second color image is applied to a second overlaid prepunched carrier sheet, is visually aligned or registered with the first separation film, and is then stripped to the second carrier in the registered position. The third and fourth separation films are registered with the others by following the same procedure. Holes which are pre-punched in the carrier sheets are placed on a pair of register pins in the contact frame to maintain the registration for the final reproduction.
The manual procedure for registering color separation films is subject to a number of shortcomings, mostly relating to accuracy. It is necessary for the stripper to register the separations to within a fraction of a row of halftone dots in each case, and this high degree of accuracy is difficult to obtain by even the most skilled and experienced strippers. In high quality printing, there are usually about 150 halftone dots per inch, and the center to center dot spacing is less than 7 mils. As can easily be appreciated, human error inevitably results from time to time, and poor quality picture reproduction often occurs. If an unacceptable registration is not discovered until press time, it is necessary to hold up the presses until the error is corrected. Press delays due to registration errors are inordinately expensive events which are to be avoided if at all possible.
Manual registration is also a tedious and time consuming process which is highly labor intensive. Typically, registration of four color separations requires between 9 and 20 minutes for a highly skilled and highly paid stripper to perform. Consequently, the labor costs contribute significantly to the overall costs. Consistency and reliability are also lacking because the quality of the registration is highly dependent upon the skill and vision of the individual stripper, and these can vary considerably. Even the same stripper cannot maintain the same level of accuracy from one job to the next due to the variations that are inherent in human performance capabilities.
Registration aids are sometimes used to magnify the halftone dot detail, but aids of this type add little to the accuracy. When a small portion of a halftone picture is magnified, it appears to the human eye to be a random collection of halftone dots rather than a recognizable picture. Therefore, picture details are often extremely difficult to align and this problem is aggravated by the difficulty the stripper has in making controlled muscle movements when viewing a magnified picture. A shaky hand or tired eyes can destroy registration, and this tendency increases with magnification. For these reasons, magnification aids are of limited utility and have not solved the problems that are inherent with manual registration.
In recent years, machines have been developed to either aid or replace the manual registration procedure. One machine is an optical-mechanical device having mechanical arms for movement of the carrier sheets and a magnified optical display for aiding the worker who performs the registration. Picture detail can be used as the basis for the registration, or special marks known as register marks can be used. The register marks are typically cross hairs located outside the field of the picture. Whether picture detail or register marks are used, this machine still relies on human skill and vision and is thus subject to many of the same problems that are associated with the strictly manual registration procedure.
Two other known machines rely on the register marks entirely in order to achieve registration. One of these machines is an electro-mechanical device having an electro-optical sensor which detects the centers of the register marks. Based on the information obtained by the sensor, the mechanical device moves one film into register with the other and then punches holes in the carrier sheets. The other machine is essentially a stripping robot that functions much like the electro-mechanical machine but tapes the separation films onto a prepunched carrier rather than acting to punch the holes itself. The basic problem with both of these latter two machines is that they rely wholly on the register marks to obtain registration. If the register marks are inaccurate, registration is also inaccurate. It is not uncommon for the register marks to be destroyed when the films are trimmed or otherwise processed, and missing registration marks render these machines completely useless. The register marks are also inaccurate at times, and this too destroys the accuracy of registration.